just like when you place too many drones, they end up going at different speeds - probably because the code can only do a certain number of commands at a time? so running at the thwump is taking one too many commands ---> drone misses a movement step/frame/whatever occasionally. i think.

The point of the map is not that one is faster than the other. If you watch - at least it appears this way - the thwump and zapper move at the same pace, until the thwump has an irregular increase in speed...this is what the map is about ;)
Pretty cool, eh?